Neither of your examples really contradict my point. If we are measuring a subset of things you need a brain for, of course factors that make your brain general

Neither of your examples really contradict my point. If we are measuring a subset of things you need a brain for, of course factors that make your brain generally less effective are going to show up on them. Lead poisoning and inadequate nutrition also decrease libido for example, and I don't think it's reasonable to conclude that this provides strong evidentiary support that libido should be our proxy for people's general capabilities and declare it equivalent to "intelligence", though there are certainly those who would try to make that case too